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O
O
O
OH O
NH
2
OHC
OH O
OH O
N
Hydrazine,
95%
O
N
N
O
H
H
MeOH,
reflux
3h
93%
DCM,
MeOH,
reflux
6h
PhI(OAc)
2
rt
89%
OH
N
N
O
N
H
O
NH
2
21
O
O
OH
Hydrazine,
94%
O
MeOH,
reflux
6h
N
N
Scheme 20. Synthesis of oxadiazole derivative 21.
O O
O
O
OO
N
H
OO
NH
2
N
H
O
N
O
Hydrazine,
95%
MeOH,
reflux
6h
O
O
methyl
4-formylbenzoate
MeOH,
reflux
6h
O O
N
N
O
O
O
O O
N
N
O
NH
O
NH
2
Hydrazine,
94%
MeOH,
reflux
6h
PhI(OAc)
2
89%
DCM,
rt
22
Scheme 21. Synthesis of oxadiazole derivative 22.
Diverse Synthetic Approaches to Design and Develop Oxadiazole-Based ... 13
post-polymerization with trifluoroacetic acid or at high temperature (190°C). The
existence of bis(hydroxyphenyl) moiety imparted high Stokes shift owing to ESIPT
(excited state intramolecular proton transfer).
3.9 Other Methods for Synthesis of 1,3,4-Oxadiazole Derivative
ȕ-Glucuronidase is an enzyme that catalyzes the transformation of ȕ-glucuronoside
to D-glucuronate and an alcohol. Taha et al. synthesized oxadiazole derivative
21 in four synthetic steps with a 94% yield (Taha et al. 2015). Methyl salicylate
formed an intermediate hydrazone while reacting in the presence of hydrazine in
methanol. The hydrazone intermediate reacted with methyl 4-formylbenzoate
and a catalytic amount of CH
3
COOH in methanol, and subsequent reaction with
PhI(OAc)
2
produced oxadiazole intermediate (Scheme 20). (Diacetoxyiodo)benzene
serves as an oxidant and facilitates intramolecular cyclization to yield oxadiazole.
The presence of the hydroxyl group on the benzene group played a major role in
inhibiting ȕ-glucuronidase.
Similarly, in another study, (diacetoxyiodo)benzene was utilized for
intramolecular cyclization by Taha et al. and synthesized oxadiazole derivative 22
(Scheme 21) (Taha et al. 2015).
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14 Oxadiazole in Material and Medicinal Chemistry
C
21
H
10
O
N
N
N
N
H
O
N
N
Br
C
21
H
10
O
Br
Cl
O
Pyridine
23
Scheme 22. Synthesis of oxadiazole derivative 23.
To develop luminescent liquid crystals, Cristiano et al. synthesized
unsymmetrical oxadiazole derivative 23 utilizing substituted tetrazole
(Scheme 22) and the corresponding 4-bromo benzoyl chloride in pyridine solvent
(Cristiano et al. 2006).
4. Conclusions
Construction of a 5-membered heterocyclic scaffold such as 1,2,4-oxadiazole in a
cost-effective process holds promising potential as it is heavily utilized as a stable
amide and ester bioisosteres in peptide mimetics. The chemical and biological
behavior of 1,2,4-oxadiazole scaffolds have good potential in drug development.
Most notable examples include commercially available drugs with a 1,2,4-oxadiazole
motif, such as oxolamine (cough suppressant), Prenoxdiazine, Fasiplon (medication
for anxiety reduction), Pleconaril (antiviral drug), Ataluren, Proxazole (analgesic
and anti-inflammatory drug), etc. Several new compounds with 1,2,4-oxadiazole
motifs are in clinical trials.
Substituted 1,3,4-oxadiazoles are considered functional surrogates of esters,
carboxamides, and acids. Also, 1,3,4-oxadiazole compounds are present in
pharmaceuticals as anti-inflammatory, anticancer, antiallergic, antimalarial, antiviral,
and several natural product-based compounds. These compounds can be prepared by
cyclizing N-acyl hydrazones using various oxidants or dehydrative cyclization using
POCl
3
, thionyl chloride, sulfuric acid, etc. Additionally, introducing visible light,
microwave technology, etc., for organic synthesis is a powerful strategy owing to
faster and more straightforward reactions.
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CHAPTER 2
1,2,4 and 1,3,4-Oxadiazole
Scaffolds in
Designing Organic
Light-Emitting Diodes
1. Introduction
Solid-state lighting materials, full-color flat panel displays, and back-lit liquid crystal
displays are some trending targets where organic light-emitting diodes are used as
raw materials. Secondly, organic light-emitting diodes or OLEDs are abundant in
mobile phones. Therefore, much research is going on to develop highly efficient,
flexible, thin displays with light weight at lower cost. Several parameters, such as
stability, energy levels, fluorescence behavior, 100% red/green/blue emission colors,
etc., are being tuned accordingly.
Apart from many heterocyclic scaffolds, oxadiazole derivatives are being used
tremendously to improve the performance of OLED devices. Materials made with
oxadiazole moiety exhibit electron-deficient characteristics, high luminescence
quantum yields, and adequate thermal stability. These kinds of available
characteristics strengthen oxadiazole-based materials toward better OLED devices
with superior electron mobility. Ideal OLED devices possess (a) a steady amount of
charge introduction from the two electrodes and (b) systematic transport of holes as
well as electrons inside luminescent layers of the device (Hughes and Bryce 2005).
Introducing electron-deficient small molecule dopants in the device can augment
the electron transport process. Similarly, the attachment of electron transporting, hole
blocking layer alongside the cathode can boost the electron transport performance of
the device. Tang and VanSlyke first published a report on OLEDs and their application
in flat panel displays in 1987 (Tang and VanSlyke 1987). Later, many heterocyclic
compounds were incorporated for tethering OLED materials with suitable optical and
electronic properties. For example, 2,5-Diaryl-1,3,4-oxadiazoles with various small
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1,2,4 and 1,3,4-Oxadiazole Scaffolds in Designing Organic Light-Emitting Diodes 19
molecules and polymers as side chains often behave as good electron transporters
(Paun et al. 2016). Alternatively, conjugated polymers such as poly(fluorene) and
poly(phenylenevinylene) are well-known hole-transporters (Hughes and Bryce
2005). This chapter summarizes OLED materials originating from two different
classes of oxadiazole systems such as 1,2,4-oxadiazole and 1,3,4-oxadiazole.
2. Organic Light-Emitting Diodes Based on
1,2,4-Oxadiazole Derivatives
Oxadiazole motifs exhibit high electron mobility and excellent thermal stability.
Materials with high triplet energy are more advantageous for energy transfer to blue
triplet emitters, hence useful while fabricating phosphorescent OLEDs (Lee et al.
2014). Li et al. developed two bipolar host materials, 1 and 2, through four synthetic
steps utilizing carbazole precursor and 1,2,4-oxadiazole as an n-type (electron
withdrawing) group (Li et al. 2014). Both the materials displayed glass transition
of more than 100°C in differential scanning calorimetry (DSC), which shows better
stability. Oxadiazole materials 1 and 2 exhibited high triplet energy at 2.71 and
2.81 eV, respectively. Development of high current efficiency of 13.0 and 16.0 cd A
–1
for the synthesized materials 1 and 2 respectively indicates good performance in
terms of blue phosphorescent OLEDs.
Taking advantage of the high triplet energies of the 1,2,4-oxadiazole motif, the
same group at Soochow University studied organic light-emitting diodes utilizing
materials 3 and 4 (Li et al. 2014). The asymmetrically positioned oxadiazole material 3
exhibited good performance with a current efficiency of 22.5 cd A
–1
, whereas material
4 displayed good performance with a current efficiency of around 19.9 cd A
–1
.
N
N
O
N
N
N
O
N
N
N
1
2
Structures 1 and 2
N
N
O
N
N
3
N
N
N
O
N
4
N
N
O
O
N
N
5
(OXD-7)
Structures 3, 4 and 5
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20
Oxadiazole in Material and Medicinal Chemistry
3. Organic Light-Emitting Diodes Based on
1,3,4-Oxadiazole Derivatives
OXD-7 or 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene is the
most common commercially available electron-transporting system based
on 1,3,4-oxadiazole units (Song et al. 2017). Tamoto et al. developed several
oxadiazole-based dimers and trimer derivatives 6–11 (Tamoto et al. 1997). The
attached triphenylamine acted as a hole transport unit, and the corresponding
oxadiazole motif served as an electron transport unit.
Wang et al. studied light-emitting properties of bis(1,3,4-oxadiazole) based
materials by utilizing poly[2-methoxy-5-(2-ethylhexoxy)-1,4-phenylenevinylene] as
emissive material (compound 12–15) (Wang et al. 2001). Fabrication with auxiliary
buffer layers might enhance the stability of the material.
N
O
N
N
CF
3
F
3
C
O
N
N
N
N
O
N
N
O
N
N
N
7
O
N
N
N
N
O
N
N
8
6
N
N
O
N
N
O
N
N
O
N
N
N
10
9
N
O
N
N
N
N
O
N
N
O
N
N
O
N
O
N
N
11
Structures 6–11
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N
N
N
N
O
O
Si
N
O
C
6
H
13
C
6
H
13
NN
17
16
Structures 16–17
1,2,4 and 1,3,4-Oxadiazole Scaffolds in Designing Organic Light-Emitting Diodes 21
N
t
B
u
t
Bu
t
Bu
O
N
H
H
O
N
N
t
Bu
12
t
Bu
O
N
N
O
N
N
13
O
O
N
N
N
N
t
Bu
t
Bu
O
N
N
N
O
N
N
t
Bu
15
14
Structures 12–15
Chan et al. utilized triphenylsilane-attached oxadiazole 16 to develop
high-performance OLEDs (Chan et al. 2002). Five different devices were constructed
using compound 16 as a blue light-emitting material. Various parameters such as
electroluminescence intensity, efficiency, current density, and blue color purity altered
significantly while varying device thickness and electron-transporting material such
as tris-(8-hydroxyquinoline) aluminum (Alq
3
). One of the devices displayed the
highest electroluminescence at 19,000 cd/m
2
. The device exhibited large external
quantum efficiency (2.4%) and small current density (674 mA/cm
2
). Interestingly,
the device can withstand temperatures over 100°C without losing luminance.
The efficiency of OLEDs can be enhanced in different ways. One such
process is through blended layers. Ahn et al. prepared an OLED device
combining an oxadiazole-based electron-transporting material 17 and
poly[2-(2-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene (MEH-PPV) film
(Ahn et al. 2004). The addition of 17 as electron transport material improved external
quantum efficiencies significantly.
Wu et al. developed a fluorene-based copolymer 18 utilizing electron-deficient
oxadiazole and electron-rich triphenylamine moiety (Wu et al. 2005). The copolymer
displayed good thermal stability and excellent glass-transition temperature (306°C).
It also exhibited bright blue emission in thin film and in a dilute solution, indicating
a promising blue light-emitting response. A high brightness of around 7,128 cd/m
2
and a high luminance efficiency of 2.07 cd/A was observed when this copolymer 18
was introduced into the OLED device. Os(fppz) is a highly efficient red-emitting
dye consisting of an osmium(II) pyridyl pyrazolate complex (Tung et al. 2004).
Incorporation of 2.6 wt % Os(fppz) complex as doped material into the OLED device
https://t.me/med1917
22 Oxadiazole in Material and Medicinal Chemistry
18
N
N
N
O
O
N
N
n
C
6
H
13
C
6
H
13
N
N
O
O
N
N
R
R
Structure 18
19
:
R
=
t
Bu
20
:
R
=
OC
12
H
25
N
N
O
O
N
N
OC
12
H
25
C
12
H
25
O
N
N
O
O
N
N
OC
12
H
25
C
12
H
25
O
C
6
H
13
C
6
H
13
21
22
Structures 19–22
with copolymer 18 enhanced brightness (28,440 cd/m
2
) and external efficiency
(9.30%) significantly.
ʌFRQMXJDWHGPDWHULDOVGLVSOD\LQWHUHVWLQJRSWLFDODQGHOHFWURQLFSURSHUWLHVDQG
hence contribute significantly to the field of organic electronics (Yuan et al. 2013;
)XNXVKLPD2\VWRQHWDOVWXGLHGʌFRQMXJDWHGPDWHULDOV±WKDWFRUUHVSRQG
to linearly designed structures (Oyston et al. 2005). These conjugated materials
were synthesized from 9,9-dihexylfluorene or spirobifluorene precursors and
aryl/diaryl-oxadiazole motifs. The authors developed OLED devices by blending
0(+339 DV HPLVVLYH PDWHULDO DQG WKH FRUUHVSRQGLQJ ʌFRQMXJDWHG FRPSRXQGV
19–22 as electron transport materials. The importance of these fabricated devices lies
in its fact that MEH-PPV contributes singlehandedly to the total electroluminescence
HYHQ LQ WKH SUHVHQFH RI ʌFRQMXJDWHG FRPSRXQGV E\ ZHLJKW +RZHYHU ULVH
LQ ʌFRQMXJDWHG FRPSRXQGV XS WR E\ ZHLJKW HQKDQFHG H[WHUQDO TXDQWXP
efficiencies more than two orders of magnitude.
In 2010, Tao et al. utilized Suzuki cross-coupling reactions to synthesize hybrid
compounds 23–25 starting from 1,3,4-oxadiazole precursor and 9,9’-spirobifluorene
https://t.me/med1917
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